In receiving devices, a low-noise amplifier is usually situated such that it is connected directly downstream of the receiving antenna. Its task is to amplify an input signal received by the antenna. In this case, the amplifier is intended to have only a low inherent noise in order thus to have a good signal/noise ratio. In addition to the requirement for a low noise figure, a switchable gain is likewise desirable. This switchable gain is called “gain step”. This prevents amplification of input signals with very high levels which overdrive the downstream signal-processing switching elements and thus cause signal distortions. In the case of such switchable gains, however, the accuracy that can be achieved is problematic since most circuit concepts have a different operating point depending on the gain. Changing the operating point in a manner dependent on the switchable gain reduces the accuracy of the “gain step”.
A further requirement is a so-called dual band combination. In this case, the receiver device is designed for the reception and the processing of signals which lie in two different frequency bands. Generally, the two frequency bands lie in the spectrum in a range which cannot be covered by a normal low-noise amplifier. In particular, CMOS amplifiers have the required impedance for matching in only a very narrow frequency range, so that the two frequency bands are not covered with just one CMOS amplifier. It is therefore expedient to provide a respective amplifier for each frequency band and to combine them in a suitable manner within the signal processing chain.
However, such combination is always associated with additional parasitic signals that influence the performance of the receiver. On the other hand, by contrast, combining the two frequency bands at a suitable location makes it possible to save chip area and thus to reduce the costs.
A further problem is improving the signal/noise ratio in low-noise amplifiers. The latter usually use field-effect transistors as input transistors IN and INX. One example of a known amplifier can be seen in FIG. 7. A similar differential amplifier is described in Tietze/Schenk “Halbleiter-Schaltungstechnik”, [“Semiconductor circuitry”], 12th Edition, Springer Verlag 2002, in Chapter 4.1.3. In order to improve the ratio at the input transistors, it is necessary to maximize the bias current flowing through the transistors. However, the use of normal nonreactive load resistors instead of the coils L1 and L2 illustrated in the example leads to a higher voltage drop across the load resistors in the case of a large DC current. As a result, the voltage range available for the transistors IN and INX is reduced and the voltage swing of the output signal OUT is reduced. It is therefore additionally necessary to increase the supply voltage, which, however, leads to additional power losses.
For this reason, an inductive load having a low quality factor is very often used instead of nonreactive load resistors. However, such an on-chip inductance requires a very large area on the chip and thus enlarges the receiver unit in a disadvantageous manner.